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Machine Punches.

287

v Punched v. Drilled Holes.< Formerly the holes were
punched in a boiler plate before rolling the latter into cylindrical
form, and alignment then obtained by very forcible use of the
drift. The holes were marked by dipping the end of a short
piece of brass tubing into white paint and transferring to the
plate; the puncher could not therefore give great accuracy, and
the plate needed considerable stretching when a pair of holes

made c half moons/ Later the centre-pop replaced the white ring,
and a * centre' punch as at B, Fig. 282, was used in the machine,
so that the hole could be punched with accuracy. The machine
punches thus took the successive forms, A, B, c, and D. c was
introduced to avoid distress of plate by giving a gradual shear,
and t>, Kennedy's spiral punch, still better carried out the idea of
c, as proved by actual tests. The bolster is shewn at E, to
support the plate while punching; and the size of hole (larger
than the punch) may be found by construction at o, a triangle